<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
    <script>
        // var 声明的变量往往会越域
        // let 声明的变量有严格的局部作用域
        /* {
            var a = 1;
            let b = 2;
        }
        console.log(a); // 1
        console.log(b); */

        // var 可以声明多次
        // let 只能声明一次
        /* var m = 1;
        var m = 2;
        let n = 3;
        // let n = 4;
        console.log(m); // 2
        console.log(n); // Identifier 'n' has already been declared */

        // var 会变量提升
        // let 不存在变量提升
        /* console.log(x);
        var x = 10; // undefined
        console.log(y); // ReferenceError: Cannot access 'y' before initialization
        let y = 20; */

        // 1. 声明之后不允许改变
        // 2. 一旦声明必须初始化，否则会报错
        /* const a = 1;
        a = 3; // Uncaught TypeError: Assignment to constant variable. */
    </script>
</body>
</html>